Pran K. Paul

Layer-transferred MoS2/GaN PN diodes

Electrical and optical characterization of two-dimensional/three-dimensional (2D/3D) p-molybdenum disulfide/n-gallium nitride (p-MoS2/n-GaN) heterojunction diodes are reported. Devices were fabricated on high-quality, large-area p-MoS2 grown by chemical vapor deposition on sapphire substrates. The processed devices were transferred onto GaN/sapphire substrates, and the transferred films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). On-axis XRD spectra and surface topology obtained from AFM scans were consistent with previously grown high-quality, continuous MoS2 films. Current-voltage measurements of these diodes exhibited excellent rectification, and capacitance-voltage measurements were used to extract a conduction band offset of 0.23 eV for the transferred MoS2/GaN heterojunction. This conduction band offset was confirmed by internal photoemission measurements. The energy band lineup of the MoS2/GaN heterojunction is proposed here. This work demonstrates the potential...

Influence of metal choice on (010) β-Ga2O3 Schottky diode properties

A systematic study of Schottky barriers fabricated on (010) β-Ga2O3 substrates is reported. Schottky barrier heights (SBHs) and current transport modes were analyzed using a combination of current-voltage (I-V), capacitance-voltage (C-V) and internal photoemission (IPE) measurements for Pd, Ni, Pt and Au Schottky diodes. Diodes fabricated for each metal choice displayed nearly ideal I-V characteristics with room temperature ideality factors ranging from 1.03 to 1.09, reverse leakage currents below detection limits and thermionic emission as the dominant current transport mode for Ni, Pt and Pd. The SBH values varied depending on the metal choice, ranging from 1.27 V for Pd and 1.54 V for Ni to 1.58 V for Pt and 1.71 V for Au, as determined using IPE measurements. Close agreement was observed between these IPE-determined SBH values and the barrier height values from I-V and C-V measurements for the Ni, Pd and Pt Schottky barriers. In contrast, for Au, a lack of general agreement was seen between the SBH me...

Investigations of metamorphic (Al)GaInP for III–V multijunction photovoltaics

We explore the lattice constant and Al content parameter space supporting (Al)GaInP materials with direct bandgaps >2.0 eV, to assess such materials for applications to future ≥4 junction multijunction photovoltaics. (AlzGa1-z)xIn1-xP test structures and prototype solar cells were grown by molecular beam epitaxy lattice-matched to either GaAsyP1-y virtual substrates or GaAs substrates over a range of Al contents and lattice constants. We observe significant anneal-induced improvements in material quality and solar cell performance in all compositions considered. Comparing test structures and solar cells with ~2.0 to ~2.1 eV bandgaps suggests that (Al)GaInP compositions with tensile-misfit (vs. GaAs) show potential to outperform lattice-matched (Al)GaInP compositions with higher Al fractions in future multijunction cells requiring top cell bandgaps >2.0 eV.

Large-area SnSe2/GaN heterojunction diodes grown by molecular beam epitaxy

We report on the synthesis and properties of wafer-scale two-dimensional/three-dimensional (2D/3D) n-SnSe2/n-GaN(0001) heterojunctions. The hexagonal crystal structure of crystalline SnSe2 grown by molecular beam epitaxy was confirmed via in-situ reflection high-energy electron diffraction and off-axis X-ray diffraction. Current-voltage (I-V) measurements of SnSe2/GaN diodes exhibited 9 orders of magnitude rectification, and the SnSe2/GaN heterojunction barrier height was estimated to be 1 eV using capacitance-voltage measurements and internal photoemission measurements. Vertical electronic transport analyzed using temperature-dependent I-V measurements indicates thermionic field emission transport across the junction. This work demonstrates the potential of epitaxial growth of large area high quality 2D crystals on 3D bulk semiconductors for device applications involving carrier injection across 2D/3D heterojunctions.

Spatial correlation of the EC-0.57 eV trap state with edge dislocations in epitaxial n-type gallium nitride

Defects in semiconductors lead to deleterious effects in electron devices, but identifying their physical sources can be difficult. An example of this in gallium nitride (GaN) high electron mobility transistors is the well-known trap state located at approximately EC-0.57 eV. This trap is strongly correlated with output power degradation and reliability issues, but despite two decades of study, its specific physical source is still unknown. To address this long-standing question, two complementary nm-resolution characterization techniques—scanning probe deep level transient spectroscopy (SP-DLTS) and electron channeling contrast imaging (ECCI)—were used to spatially map the lateral distribution of these traps and to image and characterize their relation to residual threading dislocations within NH3-MBE-grown n-type GaN. Direct comparison of the SP-DLTS and ECCI measurements on the same sample region reveals highly localized concentrations of EC-0.57 eV traps that are spatially correlated with pure edge ty...

Detecting Sub Bandgap Energies in CIGS with Electron Energy-Loss Spectroscopy

Julia I. Deitz, Pran K. Paul, Shankar Karki, Sylvain Marsillac, Aaron R. Arehart, Tyler J. Grassman, and David W. McComb 1 Dept. of Materials Science & Engineering, The Ohio State University, Columbus, OH, 43210, USA. Dept. of Electrical & Computer Engineering, The Ohio State University, Columbus, OH, 43210, USA. Dept. of Electrical & Computer Engineering, Old Dominion University, Norfolk, VA, 23529, USA.

Identifying the source of reduced performance in 1-stage-grown Cu(In, Ga)Se 2 solar cells

Traps in Cu(In,Ga)Se2 (CIGS) deposited by listage and 3-stage co-evaporation processes were characterized using conventional deep level transient and optical spectroscopies (DLTS/DLOS) and scanning-DLTS to quantify and map the deep levels associated with the different growth approaches. Two defects were observed at EV + 0.44/0.50 eV and EV + 0.97 eV. The total deep level trap concentration was found to vary primarily due to the EV + 0.97 eV trap, which was ∼4X larger in the 1-stage sample. This large increase correlates with the reduced open circuit voltage and conversion efficiency typically observed in 1-stage co-evaporation grown CIGS, suggesting that controlling the presence and concentration of this level may be important to enable high-efficiency CIGS solar cells using simpler processes.Traps in Cu(In, Ga)Se2 (CIGS) deposited by listage and 3-stage co-evaporation processes were characterized using conventional deep level transient and optical spectroscopies (DLTS/DLOS) and scanning-DLTS to quantity and map the deep levels associated with the different growth approaches. Two defects were observed at EV + 0.44/0.50 eV and EV + 0.97 eV. The total deep level trap concentration was found to vary primarily due to the EV + 0.97 eV trap, which was ~4X larger in the 1-stage sample. This large increase correlates with the reduced open circuit voltage and conversion efficiency typically observed in 1-stage co-evaporation grown CIGS, suggesting that controlling the presence and concentration of this level may be important to enable high-efficiency CIGS solar cells using simpler processes.

Comparative study of 2.05 eV AIGaInP and metamorphic GalnP materials and solar cells grown by MBE and MOCVD

This work investigates 2.05 eV bandgap (Al)GaInP alloys for use as the top junction of IMM solar cells. We explore balancing alloy composition and lattice constant as two complementary variables to achieve the target bandgap in one material system. Here both MBE and MOCVD growth methods are compared to achieve this goal. The specific compositions are Ga<inf>0.63</inf>In<inf>0.37</inf>P (tensile relaxed with respect to GaAs) and (Al<inf>0.13</inf>Ga<inf>0.38</inf>)<inf>0.51</inf>In<inf>0.49</inf>P (lattice matched to GaAs) in order to determine the relative impact of misfit and increased aluminum content, respectively. Prototype solar cell performance and defect spectroscopy (DLTS/DLOS) are used to evaluate the various alloys, and results suggest that MOCVD metamorphic Ga<inf>0.63</inf>In<inf>0.37</inf>P is promising for a high performance top cell.

Impact of the Ga/In ratio on defects in Cu(In, Ga)Se 2

The impact of the composition of a 3-stage-process deposited CIGS on the deep level spectrum was investigated using deep level transient and optical spectroscopies to probe defects throughout the bandgap to have a complete picture of the defect spectrum. It is shown that the defect spectra depend strongly on the Ga/In ratio with both higher numbers of trap levels and higher trap concentrations (by >65%) on average in the higher Ga content films. These facts can likely explain the reduced CIGS solar cell efficiency at higher Ga content.

Direct nm-Scale Spatial Mapping of Traps in CIGS

Nanometer-scale deep-level transient spectroscopy (nano-DLTS) is used to simultaneously map the spatial distribution of the EV + 0.47 eV trap in p-type Cu(In,Ga)Se2 with surface topography, providing a spatially resolved correlation between electrical traps with physical structure. It is demonstrated that the observed EV + 0.47 eV trap properties using nano-DLTS match those seen with conventional macroscopic device-scale DLTS measurements. Additionally, maps of the EV + 0.47 eV trap reveal that this trap is not uniformly distributed and is likely associated with specific grain boundary structures. The combined approach reveals overall trap impact from the local nanometer scale to the device (micrometer-centimeter) scale and correlation with physical structures on the nanometer-scale that can be broadly applied to any semiconductor material.